Our previous studies suggest that immune responses to tumor-specific antigens can be suppressed by the interaction of complementary lymphocyte clones with tumor-specific lymphocyte clones, and that the relative balance between these clones determines whether or not the host eliminates malignant cells and survives. We will study the effects of anti-idiotypic immunity on cancer development and growth using either highly immunogenic transformed cells that are rejected by normal animals or weakly immunogenic cells that grow progressively in normal animals. Three noncrossreacting fibrosarcomas of either group are available. All of these tumors have recently been induced by chronic ultraviolet light irradiation. Tumor-specific lymphocytes can be prepared in purified form. These can be used to induce (and possibly, also to destroy) anti-idiotypic immunity. Effects of anti-idiotypic immunity (anti-idiotypic antisera and T cell subpopulations) can be measured in vitro and in vivo. The effects measured in vitro include killing of tumor-specific lymphocytes by anti-idiotypic T cells, and specific unresponsiveness to stimulation with tumor cells. The effects measured in vivo are the specific unresponsiveness to stimulation with tumor cells. The effects measured in vivo are the specific breakdown of immunosurveillance of highly immunogenic potentially malignant cells permitting these cells to form tumors and specific suppression of immunoresistance after adoptive transfer of anti-idiotypic lymphocytes. Suppressson of the anti-idiotypic and/or internal image lymphocytes may lead to increased immune responses in vitro and increased immunoresistance in vivo to previously weakly immunogenic tumor cells. Together, these studies should provide a better understanding of the regulation of immunosurveillance by changes in the idiotypic network. Furthermore, the studies could lead to new ways of specifically inducing immune responsiveness to weakly immunogenic tumor cells.